Pedestrian detection system



Aug. 119,1 1,969 P. G. BARTH-:TT

PEDESTRIAN DETEC'HON SYSTEM 6 Sheets-Sheet 1 Filed March 27. 1967 ATTORNEYS P. G. BARTLETT 3,462,692 PEDESTRI'AN DETECTION' sYs'qEM.. l'

I I '6 Sheets-Sheet 2 Aug. 19, 1969.

i Filed Maron 27, 1967 PETER G. BART-LETT BY Mew/, dmy i /sady ATTORNEYSFROM AMPUFIER E (FIG l) P. G. BARTgET'T- 3,462,692 PEDESTRIANDI'IEC'IIVIOIIl SYSTEM Aug. 19, 1969*` G-Sheets- S-heet 3 Filed,lMaxjchz?. l1967 FIGB INVENTOR PETER G. BARTLETT BY MWL, www g @04,

FIG. 5

ATTORNEYS Aug. `19, v 1969 P. G. BARTLETT PEDESTRIANDETEGTION SYSTEM 6vSheets-Sheet l 4 Filed Marh- 27, 1967 1 ATTORNEYS F.l G. BARTLETT.PEDESTRIAN DETECTIO SYSTEM Aug. 19, 1969 Enea March 2T.y 1967 6Sheets-Sheet 5 5 EQ To [HLP AMPLIFIER E INTEGRATOR CIRCUIT L (FIG. 4)

FROM

DETECTOR FLATE FROM BUFFER (AMPLIFIER B als FROM AMPLIFIER X FIG. 9

INVENTOR. PETER G. BARTLETT BY MW, WW y" /orlq ATTORNEYS Aug, i9 l P.@BARRETT 3,462,692

PEDESTRIN DBTECTION SYSTEM Filed March 27, 1967 6 Sheets-Sheet 6 DETE@To R S PLATE-D DETECTOR PLATE-D l Cb vCC T EARTH GROUND INVENTOR. PETERG. BARTLETT ATTORNEYS EARTH GROUND United States Patent O U.S. Cl.328--5 7 `Claims ABSTRACT 0F THE DISCLSURE A detection system fordetecting the presence of an object, such as a pedestrian, with asensing device, such as an electrically conductive plate, exhibiting avariable capacitance with respect to earth ground. The sensing device iscoupled to one arm of a normally balanced capacitance bridge, having aninput circuit coupled to an oscillator providing a reference voltagefrequency signal relative to earth ground, and an output circuit forproviding an output frequency voltage signal when the bridge becomesunbalanced due to a change in the variable capacitance in response tothe presence of an object. A compensating device is coupled to anotherarm of the bridge for rebalancing the bridge under the control of acompensating control circuit.

The present invention is directed toward the art of object detectorsand, more particularly, to an improved self-balancing object detectorsystem.

The invention is particularly applicable for use in conjunction withdetecting the presence of pedestrians and Will be described withparticular reference thereto; although, it is to be appreciated that thesystem may be used for detecting the presence of other objects.

This invention is similar to my joint invention with Donald Henry, asdisclosed in our United States patent application, Ser. No. 613,257, ledFeb. 1, 1967, assigned to the same assignee as the present invention,and the disclosure of that application is herein incorporated byreference. That application discloses an object -detection system, whichis particularly applicable for use in conjunction with detectingmetallic objects, such as vehicles. Basically, the disclosed circuitryincludes: a sensing device in the form of an inductive loop having avariable inductive reactance which decreases in value in response to thepresence of a vehicle. An oscillator provides a reference frequencyvoltage signal for application to a normally balanced inductivereactance impedance bridge. The bridge has an input circuit forreceiving the reference frequency voltage signal, and an output circuitfor carrying an output frequency voltage signal representative of abridge unbalanced condition. The inductive loop is coupled to one arm ofthe bridge and a variable capacitor is coupled to an opposite arm of thebridge. A compensating control circuit serves, in response to a bridgeunbalanced condition, to decrease the value of the variable capacitor tothereby rebalance the bridge.

The present invention is directed toward detecting the presence of anobject, such as a pedestrian, which need not exhibit a metallic mass.This is accomplished by sensing whether the presence of an object causesa change in the capacitance between earth ground and an electricallyconductive detector plate.

Prior to the present invention pedestrian detectors utilizing thecapacitive detection principle operated with either a resonant circuitor a high impedance input circuit coupled to a power source. Both ofthese approaches suffer from the fact that resistive leakage to earthground from the detector plate reduces the systems sensitivity These andother disadvantages of previous pedestrian 3,462,692 Patented Aug. 19,1969 ice detectors are overcome by the present invention which utilizeslow impedance, non-resonant circuits.

In accordance with the present invention, the object detection systemcomprises a sensing device, such as an electrically conductive plate,having a variable capacitance with respect to earth ground, and whichcapacitance increases in value in response to the presence of an object,such as a pedestrian; oscillator means for providing a referencefrequency voltage signal relative to earth ground; a normally balancedcapacitance bridge having four bridge arms, an input circuit coupled tothe oscillator means for receiving the reference frequency voltagesignal and an output circuit for carrying an output frequency voltagesignal representative of a bridge unbalanced condition; the sensingdevice being coupled to one of the arms of the bridge so that the bridgeoutput circuit carries an output frequency voltage signal when thevariable capacitance increases in value; compensating means coupled tothe bridge for compensating for the change in the value of the variableimpedance to return the bridge to a balanced condition; and,compensating control circuit means coupled to the bridge output circuitand the compensating means for actuating the compensating means toreturn the bridge to a balanced condition in response to a bridge outputfrequency voltage signal.

In accordance with a still more limited aspect of the present invention,the compensating means includes a voltage control variable capacitancedevice connected in parallel with one ofthe arms of the bridge.

The primary object of the present invention is to provide aself-balancing object detection system which detects the presence ofobjects by sensing variations in capacitance between a sensing plate andearth ground.

It is a still further object of the present invention to provide anobject detector system which is self-balancing so as to tune out apedestrian which may be stationary in the detection zone, and thereaftercontinue to be in response to the presence of additional pedestrians.

It is a still further object of the present invention to provide adetector system wherein presence timing of the system is independent ofthe strength of the detection signal.

It is a still further object of the present invention to provide adetector system which is operated at a low frequency and in anon-resonant mode.

The foregoing and other objects and advantages of the invention 'willbecome apparent from the following description of the preferredembodiment of the invention as read in connection with the accompanyingdrawings in which:

FIGURE 1 is a block diagram illustration of the preferred embodiment ofthe invention;

FIGURE 2 is a schematic circuit diagram illustrating a capacitancebridge circuit;

FIGURE 3 is a schematic circuit diagram illustrating demodulator and adirect current differential amplifier;

FIGURE 4 is a schematic circuit diagram illustrating a direct currentamplilier, an integrator circuit, an operational timer, a switchcircuit, a threshold detector circuit and a relay driver circuit;

FIGURE 5 is a graph of wave forms of voltage versus time illustratingone aspect of the operation of the invention;

FIGURE 6 is a graph of wave forms of voltage versus time illustratinganother aspect of the operation of the invention;

FIGURE 7 is a block diagram illustrating a second embodiment of theinvention;

FIGURE 8 is a schematic illustration of a capacitance bridge circuitused in the embodiment illustrated in FIG- URE 7 FIGURE 9 is a graph ofwave forms of voltage versus time illustrating another aspect of theoperation of the invention;

FIGURE 10 is a schematic illustration of a pedestrian detector plate;

FIGURE 11 is an equivalent circuit of FIGURE 10;

FIGURE 12 is similar to FIGURE 10 with the inclusion of a pedestrian;

FIGURE 13 is an equivalent circuit of FIGURE 12; and

FIGURE 14 shows the manner of installation of the detector plate.

Referring now to the drawings, and more particularly to FIGURE 1, thereis illustrated in block diagram form a preferred embodiment of thepedestrian detector system including an oscillator A, a buffer amplifierB, an impedance 'bridge circuit C, a detector plate D, an ACdifferential amplifier E, a 90 phase shifter F, a differential squaringamplifier G, a demodulator H, a filter I, a direct current differentialamplifier J, a direct current amplifier K, an integrator circuit L, avoltage contol capacitance device M, a threshold detector N, anoperational timer O, a switch circuit T, a relay driver R, a relay CR1and a load S. Thus, the preferred embodiment of the detector systemgenerally includes: a sensing device, such as detector plate D;oscillator means including oscillator A; a normally balanced capacitancebridge C, having four arms, one being coupled to the plate D, an inputcircuit coupled to oscillator A through amplifier B; compensating meansincluding the voltage controlled capacitance device M; compensatingcontrol circuit means including circuits E, F, G, H, I, J, K, L, N, Oand P; and, a detector output means including relay driver R, relay CR1and load S.

Oscillator A may take various forms, but preferaby includes a standardtransistorized Colpitts oscillator which provides, at its outputcircuit, a low frequency reference voltage signal of sinusoidal waveform and of around four volts peak to peak. The buffer amplifier Bserves as a buffer stage between oscillator A and the phase shiftercircuit F and bridge circuit C. Bridge circuit C, as will be describedin greater detail with reference to FIGURE 2, takes the form of acapacitance bridge having four arms, of which one is coupled both to adetector plate D as well as to the votage controlled capacitance deviceM. The bridge output circuit is coupled to an AC differential amplifierE. The output circuit of amplifier E is coupled to a demodulator circuitH, to be described in greater detail hereinafter with reference toFIGURE 3. The output circuit of buffer amplifier B is coupled to a phaseshifter circuit F, which serves to shift the reference frequency signalby 90, lagging. The shifted reference signal is then applied to adifferential squaring amplifier G, which may take any suitable form andserves to square up the sinusoidal output wave from the oscillator in asymmetrical fashion, so that the square wave output signal is 90 out ofphase and lagging with respect to the oscillator output signal. Thesquare wave signal is then applied to demodulator H, where the bridgeoutput signal is demodulated by the squared up and phase shiftedreference signal to obtain an alternating signal indicative of theincrease in the capacitance reactance between plate D and earth ground.The low frequency portion of this output signal is passed by filter I asa direct current voltage, having a value representative of the change inthe capacitive reactance. The output of filter I is amplified by thedirect current differential amplifier J, to be described in detailhereinafter with reference to FIGURE 3. The output of differentialamplifier .T is amplified by a DC amplifier K and integrated byintegration circuit L. The amplifier output signal of amplifier K isapplied to threshold detector N, where, if the signal is sufiicientlygreat, a relay driver R is actuated to de-energize normally energizedrelay CRI coupled to load S. The output of threshold detector N is alsocoupled to an operational timer O, which, after a predetermined periodof time, actuates a switch circuit P which serves to actuate theintegrator circuit L. The integrator circuit L, when actuated, serves toapply a voltage to the voltage controlled capacitor device M, which, inturn, serves to rebalance bridge circuit C, to thereby tune out thechange in the capacitive reactance. Circuits K, L, N, O, P, device M,relay driver R, relay CR1 and load S are described in greater detailhereinafter with reference to FIGURE 4.

DETECTOR PLATE The detector plate D, as shown in FIGURES 10, 12 and 14,preferably takes the form of an electrically conductive wire meshscreen. If desired, the plate may take other forms, such as anelectrically conductive, solid plate. The plate is installed on asidewalk or a roadbed so that it generally defines a horizontal plane.Normally, as shown in FIGURE 14, plate D is installed in a dielectricmedium, such as concrete, which overlies earth ground. In the absence ofa pedestrian, plate D exhibits a capacitance Ca with reference to earthground potential, as represented by the equivalent circuit shown inFIGURE 11. The value of capacitance Ca is dependent on such factors asthe distance and dielectric medium between the plate and reference levelof earth ground potential. As shown in FIGURE 12, when a pedestrianwalks over plate D, the total capacitance increases by the Value of thecapacitance Cb between the plate and the pedestrian, as well as thecapacitance Cc between the pedestrian and earth ground. The equivalentcircuit, during the presence of a pedestrian, is shown in FIGURE 13.

CAPACITIVE IMPEDANCE BRIDGE CIRCUIT The impedance bridge circuit C, asschematically illustrated in FIGURE 2, includes four bridge arms 10, 12,14 and 16. Arm 10 includes a variable resistor 26. Arm 12 includes afixed resistor 22. Arm 14 includes a normally conductive, constantcurrent source, NPN transistor 25 having its base connected to a C+voltage supply source through resistor 27, and its emitter connected toground through series resistors 31 and 32. A relatively small capacitor34, which may be on the order of 0.0027 microfarad, is connected betweenthe collector of transistor 25 and ground. A fixed resistor 36 and abridge balancing resistor 38 are connected together in series acrosscapacitor 34. Arm 16 includes a normally conductive, constant currentsource, NPN transistor 28 having its base connected to a C-ivoltagesupply source through resistor 29 and its emitter connected to earthground through series connected resistors 30 and 32. A large capacitor33, which may be on the order of 0.1 microfarad, is connected betweenthe collector of transistor 28 and the detector plate D. A largecapacitor 40, which may be on the order of 0.01 microfarad, couples thejunction of capacitor 33 and collector of transistor 28 with the cathodeof a Zener diode 42, which serves as a voltage controlled variablecapacitance device. The junction capacitance of Zener diode 42 is small,such as on the order of 0.001 microfarad, relative to capacitor 33. Thejunction of the cathode of Zener diode 42 and capacitor 40 is connectedthrough a resistor 44 to a B+ voltage supply source. The junction ofresistors 22 and 26 is connected to ground through a capacitor 46, aswell as to the B+ voltage supply source through a resistor 48. Resistor26 is connected in series with transistor 2S by means of an NPNtransistor 50, having its collector connected with resistor 26 and itsemitter connected to the collector of transistor 25. Similarly, resistor22 is connected in series with transistor 28 by means of an NPNtransistor 52, having its collector connected with resistor 22 and itsemitter connected to the collector of transistor 28. The base oftransistor 50 and the base of transistor 52 are connected together incommon so that the input circuit of the bridge is taken between groundand the common base connection of transistors 50 and 52. The Ibridgeinput circuit is coupled across the output circuit of buffer amplifier Bfor receiving therefrom a sinusoidal alternating current frequencysignal. The output circuit of bridge C is taken between the collectorsof transistors 50 and 52 and is coupled to amplifier E through couplingcapacitors 54 and 56.

DEMODULATOR Demodulator H, as schematically illustrated in FIG- URE 3,includes a differential amplifier 58 having a demodulating PUPtransistor 60 connected across its output circuit. Amplifier 58 includesa pair of NPN transistors 62 and 64, having their emitters connectedthrough resistors 66 and 68 to the collector of a normally conductive,constant current source, NPN transistor 70. Transistor 70 has itsemitter connected to ground through a resistor 72 and its base connectedto the C+V voltage supply source. The base electrodes of transistors 62and 64 are connected through resistors 74 and 76, respectively, to a D+Ivoltage supply source. Preferably, the C+ voltage supply source is onthe order of +4 volts and the B+ voltage supply source is on the orderof {+24 volts, while the D+' voltage supply source being at a positivevalue between the other two sources. The collector of transistor 62 isconnected through a resistor 78 and a resistor `80 to the Br-lvoltagesupply source. Similarly, the collector of transistor 64 is connectedthrough a resistor 82 and then through a resistor 80 to the B1+' voltagesupply source. Similarly, the collector of transistor 64 is connectedthrough a resistor 82 and then through a resistor 80 to the B+v voltagesupply source. The junction of resistors 78 and 82 is connected toground through a capacitor 84. PNP transistor 60 has its emitterconnected to the collector of transistor 64 and its collector connectedto the collector of transistor 62. The output from amplifier G, FIGURE1, is applied to the base of transistor 60 so that a square Wave,shifted 90 lagging with respect to the reference frequency signalprovided by oscillator A, is applied to transistor 60. The input circuitto differential amplifier 58 is taken between collectors 62 and 64 andis connected across the output circuit of the AC differential amplifierE. The output circuit of demodulator H is taken across the collectors oftransistors 62 and 64 and applied to the filter I.

FILTER AND AC DIFFERENTIAL AMPLIFIER The filter I includes a capacitor85 connected across the output circuit of demodulator H throughresistors 86 and 88. Differential amplifier J, as schematicallyillustrated in FIGURE 3, includes a pair of PNP transistors 90 and 92,having their emitters connected in common and thence through a resistor94 to a B+ voltage supply source. The collector of transistor 90 isconnected through a resistor 96 to ground and the collector oftransistor 92 is connected through a resistor 98 to ground. The outputcircuit of differential amplier J is taken across the collectors oftransistors 90 and 92 and applied to amplifier K.

DC AMPLIFIER DC amplifier K, as schematically illustrated in FIG- URE 4,includes an NPN transistor 100 and a PNP transistor 102, having theiremitters connected together through a resistor =104. The collector oftransistor 100 is connected to the Bf+ voltage supply source and thecollector of transistor 102 is connected through a resistor 106 toground. The base of transistor 100 is connected through a resistor 108to the collector of transistor 92 in amplifier J, and the base oftransistor 102 is connected through a resistor 110 to the collector oftransistor 90 in amplifier I The output of amplifier K is taken betweenground and the collector of transistor 102, and is applied to theintegrator circuit L as well as to the threshold detector circuit N.

6 INTEGRATOR CIRCUIT The integrator circuit L, as schematicallyillustrated in FIGURE 4, includes an inverter NPN transistor 112, havingits emitter connected to ground and its base connected by a resistor 114to the collector of transistor 102 in circuit K. The collector oftransistor 112 is connected through a resistor 116 to the BH-l voltagesupply source. Circuit L also includes a pair of NPN transistors 118 and120. Transistor 118 has its collector connected to the collector oftransistor 112 and its base connected through a resistor 122 to theswitch circuit P, as described in greater detail hereinafter. A diode124, poled as shown in FIGURE 4, is connected across the collector toemitter circuit of transistor 118. The emitter of transistor 118 is alsoconnected to ground through a resistor 126y and a capacitor 128.Transistor has its emitter connected through a resistor to the junctionof capacitor 128 and resistor 126 and, thence, through a single pole,double throw switch SW-2 to the anode of the voltage controlledcapacitance device M taking the form of Zener diode 42. Capacitor 128 isrelatively large, such as on the order of 100 microfarads, relative tocapacitors 34 and 40, and the capacitance of Zener diode 42. Transistor120 also has its collector connected to a collector of transistor 112and its base connected through a resistor 132 and the anode-cathodecircuit of a diode 134, poled as shown in FIGURE 4, to the thresholddetector circuit N, to be described in greater detail hereinafter.Switch SW-2, which is used for calibration purposes, connects the anodeof Zener diode 42 with the junction of capacitor 128 and resistor 126.More particularly, switch SW-2 includes a movable contact 127 connecteddirectly to the anode of Zener 42, a stationary contact 129 connected tothe junction of capacitor 128 and resistor 126, and a second stationarycontact 131. Stationary contact 131 is connected to the junction ofseries connected resistors 133 and 135, which define a voltage dividercircuit connected between ground and the B-lvoltage supply source.

THRESHOLD DETECTOR The threshold detector N, as schematicallyillustrated in FIGURE 4, preferably takes the form of a Schmitt 'triggercircuit, including a pair of NPN transistors 136 and 138 having theiremitters connected together in common, and then through a resistor 140to ground. The collector of transistor 136 is connected through aresistor 142 to the B+ voltage supply source and the collector oftransistor 138 is connected through a resistor 144 to the B+, voltagesupply source. The collector of transistor 136 is also connected to thebase of transistor 138 through a resistor I146. The junction of the baseof transistor 138 and resistor 146 is connected through a resistor 148to ground. The input circuit of the threshold detector is taken betweenground and the base of transistor 136, which is connected to thecollector of transistor 102. The output circuit of the thresholddetector is taken between ground and the collector of transistor 138,which collector is connected to the cathode of diode 134 in theintegrator circuit L, as well as to the relay driver circuit R and theoperational timer O.

OPERATIONAL TIMER The operational timer O, as schematically illustratedin FIGURE 4, is a linear ramp function generator in the form of anoperational DC amplifier having negative feedback and generallycomprises PNP transistors 150, 152 and 154; an NPN transistor 156; anegative feedback capacitor 158; and, an output resistor 160. Transistor150 has its base connected to the collector of transistor 138 in thethreshold detector circuit N, and its emitter connected to the B+voltage supply source. 'Ihe collector of transistor 150 is connected tothe collector of transistor 156. Transistor 152 has its emitterconnected to the B+ voltage supply source and its base connected througha variable timing resistor 162 to the junction of resistors 164 and 166,which are connected between ground and the B+` voltage supply source.The collector of transistor 152 is connected through a diode 168, poledas shown in FIGURE 4, to the base of transistor 156. The emitter oftransistor 156 is connected to the base of transistor 152 through acapacitor 170. Transistor 156 also has its collector connected through aresistor 172 to the B+' voltage supply source, and its emitter connectedto the junction of resistors 174 and 176, which serve as a potentialdivider connected between ground and the B+ voltage supply source forpurposes of lowering the collector voltage of transistor 152. Transistor154 has its base connected to the collector of transistor 156 and itsemitter connected to the B+ voltage supply source. The collector oftransistor 154 is connected to ground through the output resistor 160.The junction of resistor 160 and the collector of transistor 154 isconnected to the base of transistor 152 through the timing capacitor158. Also, a capacitor 178 is connected between the junction ofresistors 174 and 176 and the collector of transistor 154.

SWITCH CIRCUIT The switch circuit P, as schematically illustrated inFIGURE 4, includes a Zener diode 180 and an NPN transistor 182. Thecathode of Zener diode 180 is connected through a resistor 184 to thejunction of resistor 160 and the collector of transistor 154. The baseof transistor 182 is connected to the anode of Zener diode 180.Transistor 182 also has its emitter connected to ground and itscollector connected to the BH- voltage supply source through a resistor186. Also, the junction of resistor 186 and the collector of transistor182 is connected through a diode 188, poled as shown in FIGURE 4, to thebase of transistor 118 in the integrator circuit L.

RELAY AND RELAY DRIVER The relay driver circuit R includes a PNPtransistor 190 and a single pole, double throw mode switch SW-l. Modeswitch SW-l includes a movable contact 192 and a pair of stationarycontacts 194 and 196. Contact 196 is connected to ground and contact 194is connected to the B+ voltage supply source through a resistor 198.Also, contact 194 is connected to the collector of transistor 138 in thethreshold detector circuit N. Transistor 190 has its emitter connectedto the B+ voltage supply source and its base connected through acapacitor 200 to the stationary contact 194. Also, the base oftransistor 190 is connected to the movable contact 192 of mode switchSW-l through a resistor 202. The collector of transistor 190 isconnected to ground through relay coil CR1-C of relay CR1. A diode 204,poled as shown in FIGURE 4, is connected across the coil to protecttransistor 190 from the large inductive surge when relay CR1deenergizes. Relay CR1 also includes a pair of normally closed contactsCR1-1, which may serve to connect load S across an alternating currentvoltage source V.

OPERATION With power applied to the circuit, the buffer amplifier Bapplies a sinusoidal low frequency, alternating current voltage signalto the impedance bridge circuit C. This signal is applied to the commonbases of transistors 50 and 52. During each positive half cycle of theinput signal, transistors 50 and 52 are biased into conduction,whereupon current tlows from the B+ voltage supply source through onepath including bridge arm 10, transistor 50, ybridge arm 14 to ground,and through a second path in cluding bridge avm 12, transistor 52 andthrough bridge arm 16. The bridge circuit is initially balanced byactuating switch SW-Z (FIGURE 4) so that Zener diode 42 is connectedfrom the junction of voltage divider resistors 133 and 135 to thejunction of capacitor 40 and resistor 44 in the circuit of FIGURE 2.Variable resistor 26 is then varied so that the bridge is balanced andno output voltage appears between the collectors of transistors 50 and52. Variable resistor 38 is adjusted to balance out the effectiveresistance of the sensing plate D refiected into the impedance bridgecircuit.

As a pedestrian walks over plate D (FIGURES 12 and 13), the totalcapacitance between the plate and earth ground increases in value. Thevalue of this total capacitance, however, is on the order of 0.0005microfarad. As stated previously, the value of capacitor 33 isrelatively large, on the order of 0.1 microfarad. Thus, an increase inthe value of this small plate capacitance results in a net increase inthe capacitance Ibetween the collector of transistor 28 and earthground. Accordingly, the capacitive reactance of arm 16 has decreased,increasing the value of the current flow through arms 12 and 16 withrespect to that fiowing through arms 10 and 14. The bridge becomesunbalanced and an alternating current voltage of some value appearsacross the collectors of transistors 50 and 52, which is applied throughcoupling capacitors 54 and 56 to the AC differential amplifier E. Thebridge may be rebalanced by lowering the value of the capacitance of arm16. In parallel with arm 16 there is provided a capacitor series circuitincluding capacitor 40, Zener diode 42, as well as capacitor 128 (FIGURE4). Since the cathode of Zener diode 42 is connected through resistor 44to the B+ voltage supply source, its capacitance can be changed byvarying the value of the potential stored by capacitor 128. Thus, forexample, if the direct current voltage stored by capacitor 128 isdecreased so as to drive the anode end of Zener diode 42 morenegatively, then the voltage applied across the anode to cathode circuitof the diode will increase. As is well known, one characteristic of aZener diode is that it serves as a voltage controlled variablecapacitance device, wherein its capacitance varies inversely with thedirect current voltage applied across its anode-cathode circuit. As willbecome more apparent from the description which follows, thecompensation control circuit means, including circuits F, G, H, I, J, K,L, M, N, O and P, serves to lower the voltage stored by capacitor 128 sothat the voltage applied across diode 42 increases. This, in turn,decreases the junction capacitance of Zener diode 42. Since diode 42 isin parallel with capacitor 34, the capacitance of `bridge arm 16 isdecreased. This decrease in the capacitance of arm 16 increases theimpedance of that arm so that the bridge is rebalanced. The operationrcquired by the control circuit for applying the necessary controlvoltage to Zener diode 42 is discussed below.

The reference frequency voltage signal provided by oscillator A isapplied through the buffer amplifier B to phase shifting circuit F.Circuit F serves to shift the frequency signal so that it is out ofphase and lagging that provided by the oscillator. This is a sinusoidalsignal which is applied to the differential squaring amplifier G, whichsquares up the sinusoidal wave in a symmetrical fashion so as to obtaina square wave output voltage signal, as shown by the wave form VG inFIGURE 5, which is of the same frequency and is out of phase by 90,lagging, with respect to the oscillator output voltage, as representedby wave form VA in FIGURE 5.

When the bridge is unbalanced, due to the presence of a pedestrian, itsoutput frequency voltage signal is amplified by an AC differentialamplifier and applied to the demodulator H. This signal, as representedby the wave form VE in FIGURE 5, is shifted in phase by 90 lagging, withrespect to that of the oscillator output signal VA. This signal isapplied to the demodulator H where it is demodulated with the outputfrom the differential squaring amplifier G. With reference to FIGURE 3,it will be noted that the output from amplifier E is applied to thebases of transistors 62 and 64 so that an output voltage of the samefrequency is obtained across the output circuit of the differentialamplifier, as taken between the collectors of transistors 62 and 64.However, the output from amplifier G, which is of the same frequency asthe output from amplifier E and of the same phase (since 'both areshifted 90 in the same sense with reference to the output from theoscillator A), is applied to the base of demodulating transistor 60.Accordingly, during each negative half cycle of the output signal fromamplifier G, transistor 60 is forward biased into conduction to shortcircuit the collectors of transistors 62 and 64. Accordingly, only thepositive alteration of the output voltage of differential amplifier 58will appear across the output circuit of demodulator circuit H, asindicated by wave form VH in FIGURE 5. This signal is applied throughresistors 86 and 88 to the filter capacitor 85 which serves to filterthe signal and provide a direct current voltage, as represented by thewave form VI in FIGURE 5. This voltage is amplified by the DCdifferential amplifier J and the output voltage appearing across thecollectors of transistors 90 and 92 is a differential voltage signaldirectly proportional to the capacitive change. This differential signalis again amplified by DC amplifier K (FIGURE 4) which lprovides anoutput vvolt age between ground and the collector of transistor 102,which is directly proportional to the capacitance.

It will now be appreciated that any change in the effec tive resistanceof the detector plate will not be detected by demodulator H and lter I.More particularly, any resistive component of the bridge output voltagewill be in phase with the oscillator voltage, as represented by waveform VA in FIGURE 5. Thus, the resistive component bridge output voltagewill be 90 out of phase with respect to the voltage output of amplifierG, see wave form VG in FIGURE 5. Accordingly, the output voltage ofdemodulator H will have equal positive and negative alterations, asopposed to that shown in Wave form VH, which will be completely filteredout by the filter capacitor 85. Thus, the output voltage of DC amplifierJ represents only a capacitive reactance change and not a change in theeffective resistance of the detector plate.

The output voltage of the DC amplifier K is applied both to theintegrator circuit L as well as to the threshold detector circuit N. Incircuit L the output voltage signal of circuit K is inverted bytransistor 112 and applied through diode 124 and transistors 118 and 120to the capacitor 128. Thus, the voltage, as integrated and stored bycapacitor 128, is directly proportional to the inductance of the sensingloop. The base to emitter drop of transistor 112 serves to maintain thecollector voltage of transistor 102 at some reference value, such asaround 1 volt. When the presence of a pedestrian is detected, anincreased voltage is applied to the base of transistor 112, attemptingto discharge capacitor 128. As discussed previously, when the voltageacross capacitor 128 is decreased, there will result an increasedvoltage drop across the Zener diode 42, thereby decreasing itscapacitance. By decreasing the capacitance of Zener diode 42, the bridgeis rebalanced and a detected pedestrian is tuned out. However, capacitor128 is not permitted to discharge to rebalance the bridge circuit untilforward rbiasing potentials are applied to transistor 118 or 120, bothlocated in parallel discharge paths for capacitor 128.

The output voltage of DC amplifier K is applied between ground and baseof transistor 136 in the threshold detector circuit N. Normally, thissignal is at some reference value, such as 1 volt, as discussedhereinabove. However, this voltage increases in value upon detection ofa pedestrian, and when the value of the increased voltage is sufficientto force transistor 136 into conduction, the collector voltage of thattransistor decreases and begins to turn off transistor 138. Theregenerative action of the Schmitt trigger coupling forces thetransistors to change -state so that transistor 138 is reversed biased.Thus, the collector voltage of transistor 138 approaches the value ofthe B+ voltage supply source to reverse bias transistor 120 in theintegrator circuit L. Since transistor `118 is also reversed biased,capacitor 128 is prevented from discharging and therefore cannot at thistime serve to rebalance the bridge circuit.

The output voltage of the threshold detector circuit N is taken betweenground and the collector of transistor 138 and is also applied to therelay driver circuit R. Since the collector voltage of transistor 138 isapproaching the value of the B+ voltage supply source, its voltage isreected through resistor 202 to the base of transistor 190 in relaydriver circuit R to reverse bias this transistor, and thereby deenergizerelay CRI. Accordingly, relay contacts CRI-1 open and remain open untiltransistor 190 is forward biased.

The posltive collector voltage of transistor 138 is also applied to thebase of transistor in the operational timer O to reverse bias thistransistor. This permits the operational timer to commence its timingfunction. Previous to a pedestrian detection, transistor 150 is forwardbiased since the output volta-ge of the threshold detector is at somevalue approaching ground potential. Thus, in this condition transistor150 applies substantially a B+ voltage signal to the base of transistor154 so that this transistor is reversed biased. Accordingly, the outputvoltage across is substantially at ground potential. Thus, the negativeside of capacitor 158 is referenced to ground potential and thecapacitor is charged `by the current flowing through the emitter to baseof transistor 152. The value of the charge stored by capacitor 158 is,therefore, essentially the value of the B+ potential minus the voltagedrop between the emitter and base of transistor 152.

When transistor 150 becomes reversed biased, upon detection of apedestrian, transistor 154 becomes conductive. A potential is developedacross the adjustable timing resistor 162 and the value of thispotential is determined by the potential stored by capacitor 158 as wellas the setting of the potential divider comprised of resistors 164 and166. Capacitor 158 commences to discharge through resistors 162, 164 andoutput resistor 160, thereby slightly decreasing the potential stored bythe capacitor. As the value of the potential stored by the capacitordecreases, transistor 152 begins to conduct. Current flows from the B+voltage supply source through the emitter to collector of transistor 152to the base of transistor 156 through diode 168, causing transistor 156to conduct. As transistor 156 begins to conduct, the potentialappearin-g on its collector decreases toward ground potential. As thisoccurs, transistor 154 begins to conduct more heavily. Current,therefore, commences to ow from the emitter to collector of transistor154, whereupon a voltage begins to build up across the output resistor160. This voltage increase increase across resistor 160 lifts thepotential on the negative end of capacitor 158 to a point above groundpotential. Since the voltage across lcapacitor 158 cannot changeinstantaneously, the potential at the positive end of the capacitor isalso raised instantaneously the same amount as on the negative end. Thistends to turn off transistor 152, 156 and 154. Capacitor 158 thendischarges through resistors 162, 164 and 160. Again, transistors 152,156 and 154 begin to conduct, applying more potential across the outputresistor 160. The circuit continues to function in this manner so thatfor a square wave input a linear ramp function is produced acrossresistor 160. This voltage increases in a linear manner with elapsedtime from ground potential toward the value of the B+ voltage supplysource, as indicated by the wave form V0 illustrated in FIGURE 6. Solong as the operational timer O is not reset, as by a momentaryapplication of a positive signal from the output circuit of thresholddetector N to the base of transistor 150, this linear ramp function willcontinue until transistor 154 is completely saturated. The slope of thewave form of the output voltage, as represented by wave form V0 inFIGURE 6, is determined by the adjustment of resistor 162. If theresistance of resistor 162 is varied to the point that it issubstantially zero resistance, the output voltage will very quicklyapproach the value of the B+ voltage supply source, as indicated by waveform Vo in FIGURE 6.

The output voltage of operational timer O is applied to Zener diode 180.After a predetermined period of time, as represented by T1 in FIGURE 6,the output voltage of operational timer O will obtain a value exceedingthe break over voltage, represented by the wave form VZ in FIGURE 6, ofthe Zener diode 180. As this occurs, transistor `182 is forward biasedinto conduction so that its collector potential is referenced to groundpotential. As will become apparent from the description which follows,when transistor 182 is forward biased into conduction, the integratorcircuit L is actuated so that capacitor 128 may discharge and therebyrebalance the capacitance bridge circuit C to tune out the change in thecapacitive reactance of thedetector plate. During this predeterminedtime period, relay CR1 is deenergized to indicate the presence of adetected pedestrian. However, since a detected pedestrian may becomestationary over the detector plate, it is necessary to rebalance thebridge circuit after some given vperiod of time so that the circuit candetect additional pedestrians walking over the detection plate. The timeperiod may be adjusted by adjustable resistor 162 to obtain a desiredtime range, such as from 100 milliseconds to minutes. The minimum timerange, i.e., for example, 100 milliseconds, is accomplished by adjustingthe variable resistor 162 so that substantially all of the resistance isshunted out and this is normally used only for a pulse mode operationwhen it is desired to count medestrians as opposed to obtaining anoutput signal which has a duration representative of the period of timethat a pedestrian is located over the detector plate. For the pulse modeoperation, switch SW-l has its movable contact 192 connected tostationary contact 196. With this adjustment and with the notedadjustment of resistor 162, the bridge circuit is almost immediatelyrebalanced, as indicated by time T2 in FIGURE 6, which, for example, mayoccur around 100 milliseconds after the pedestrian has been detected.Also, resistor 202 and capaictor 200 in the relay driver circuit form atime constant circuit permitting the relay to be deenergized for aperiod of around 100 milliseconds.

After the operational timer O has completed its timing function toforward bias transistor 182 into conduction, the collector of thistransistor is referenced to ground potential. Similarly, the base oftransistor 118 in the integrator circuit L is also referenced toward thevalue of ground potential through resistor 122 and the anode to cathodecircuit of diode 188. Transistor 118 is therefore forward biased intoconduction so that capacitor 128 can discharge through the emitter tocollector of transistor 118 and through the collector to emitter oftransistor 112 t in accordance with the increase in capacitive reactanceof the sensing loop. As discussed previously, this decrease in thevoltage stored by capacitor 128 increases the voltage applied acrossZener diode 42, so that the capacitance of this diode decreases torebalance bridge circuit C. As

the bridge becomes rebalanced, the collector voltage of transistor 102in the DC amplifier K returns to its reference level of substantially 1volt, which is not sufficient to maintain the threshold detector circuitN actuated. Accordingly, transistor 138 in the threshold detectorcircuit reverts to its normally conductive state and again forwardbiases transistor 120 in the integrator circuit L and also forwardbiases transistor 190 in the relay driver circuit, thereby energizingrelay CR1. The detector circuit is now in condition to detect asucceeding pedestrian.

SECOND EMBODIMENT Reference is now made to the ock diagram of FIG- URE7, illustrating a second embodiment of the invention, which is quitesimilar to that illustrated in the block diagram of FIGURE 1, and,accordingly, only the variations therefrom will be described in detailwith like components in both figures being identified with likecharacter references. As discussed previously with reference to FIGURE2, the effective resistance of the sensing plate D is initially balancedout with a manually adjustable resistor 38 located in arm 14 of theimpedance bridge. In accordance with this second embodiment of theinvention, resistors 36 and 38 are eliminated and, instead, bridgecircuit C, to be discussed in greater detail hereinafter with referenceto FIGURE 8, includes a voltage controlled variable resistance deviceshown as block T in FIGURE 7. In addition, this second embodiment of theinvention includes a second differential squaring amplifier U whichserves the same function as the differential squaring amplifier Gdiscussed in detail with reference to FIGURE l. The output of thedifferential squaring amplifier U is coupled to the output of the ACdifferential amplifier E. The demodulator V is constructed in the samemanner as the demodulator H, discussed in detail hereinbefore withreference to FIGURE 4. The output circuit of demodulator V is coupled toa filter circuit W, which takes the same form as filter I discussed indetail hereinbefore with reference to FIGURE 3. The output of filter Wis coupled to the input circuit of a second DC differential amplifier X,which takes the form as discussed hereinbefore with reference to thedifferential amplifier J illustrated in FIG- URE 3. The output of thedifferential amplifier X is applied to the voltage controlled resistancedevice T.

The impedance bridge circuit C', as schematically illustrated in FIGURE8, is quite similar to bridge circuit C illustrated in FIGURE 2 and,accordingly, only the variations therefrom will be described in detailwith like character references being used in both figures foridentifying like components. As will be appreciated from a comparisonwith FIGURE 2, FIGURE 8 differs by its elimination of resistors 36 and38, and by its addition of a voltage controlled variable resistancedevice T, which includes a field effect transistor 210, as well as aIresistor 212 and a pair of capacitors 214, 216 and a resistor 218. Thefield effect transistor 210 has source electrode 220 connected to thecollector of transistor 25 through resistor 212, and its drain electrode222 connected to lthe base of transistor 25 as well as to the C+ voltagesupply through resistor 27. Thus, electrodes 220 and 222 are in parallelwith resistors 31 and 32 in the emitter circuit of transistor 25. Thegate electrode 224 of the field effect transistor 210 is connected tothe junction of capacitors 214 and 216 which form a series circuitbetween ground the common connection between basis of transistors 50 and52. Resistor 218 is connected between the junction of capacitors 214 and216 and amplifier X. Amplifier X, as discussed hereinbefore, isconstructed as DC differential amplifier J illustrated in FIGURE 3.Thus, with reference to FIG- URE 3, amplifier X includes a resistor 98taken between ground and the collector of transistor 92. The output ofamplifier X is taken across resistor 98 and, accordingly, only one leadis taken from amplifier X, at the collector of transistor 92, and thislead extends to resistor 218 in the circuit shown in FIGURE 8.

OPERATION Bridge circuit C', as illustrated in FIGURE 8, is initiallybalanced so that no output frequency signal is obtained across itsoutput circuit, as'taken between the collectors of transistors 50 and52, by adjusting the Value of adjustable resistor 26 in arm 14. Theeffective resistance of the sensing plate, such as the leakageresistance to earth ground, will change in value with variation intemperature and moisture as well as other conditions, such as the typeof cable connection to the detector circuitry and the dielectric mediuminvolved. Accordingly, the effective resistance is an unstable andunpredictable parameter. The variations in the effective resistanceunbalance the bridge, and the bridge is rebalanced by applying a voltagebetween ground and the gate electrode 224 of the filed effect transistor210. As is well known, the resistance between the source electrode andthe drain electrode of a field transistor varies in proportion to thevoltage applied to the gate electrode. Accordingly, if the effectiveresist- 13 ance of the detector plate D has varied in value, the bridgeis rebalanced by a compensating change of the resistance in arm 14. Thismay be accomplished by applying a Voltage proportional to the resistancechange to the -iield elfect transistor to vary the resistance betweenthe source and drain electrodes.

When the effective resistance of the detector plate D varies, the bridgecircuit C becomes unbalanced. Accordingly, an output frequency signal isprovided across the output circuit of bridge circuit C'. Any variationin the capacitive reactance is sensed by demodulator H and filter I,described hereinbefore. Va-riations in the effective resistance aresensed by the demodulator V and iilter W. A change in effectiveresistance is reected into the bridge as an amplitude change in phasewith the oscillator signal. Thus, the amplified bridge output voltage,as represented by the wave form VE, in FIGURE 9, due to this resistivecomponent is in phase with the oscillator frequency signal, representedby wave form VA in FIGURE 9. The output of oscillator A is applied tothe buffer amplifier B and then to the differential squarin g amplifierU, which squares up the sinusoidal wave in a symmetrical fashion, asshown by the wave :form VU in FIGURE 9. The demodulator circuit V issimilar to the demodulator circuit H, described in detail hereinbeforewith reference to FIGURE 3. The output voltage of the demodulatorcircuit V is an alternating signal of positive pulsations, asrepresented by the wave form Vv in FIGURE 9. In the same manner asdiscussed previously with respect to demodulator H and iilter I, theoutput of demodulator V is applied to a filter W, with the output of thefilter taking the form of a direct current voltage, as represented byWave Iform VW in FIGURE 9. This direct current voltage is then amplifiedby the DC differential amplifier X, which takes the form of amplifier Jillustrated in FIGURE 3. The output of this differential amplifier istaken at the collector of transistor 92 (see FIGURE 3) and is a directcurrent voltage signal directly proportional to the change of theeffective resistance of the sensing loop. This signal is then applied tothe field effect transistor 210, as illustrated in FIGURE 8, acting as avoltage controlled variable resistance to rebalance the bridge circuit.Accordingly, the detector system illustrated in FIGURE 7 serves torebalance the bridge as variations in the effective resistance occur.Thus, the system detects the presence of pedestrians only by respondingto variations in the capacitive reactance. The operation of this phaseof the system has been described hereinbefore in detail with referenceto the embodiment shown in FIG- URE l.

The invention has been described in connect-ion with a particularpreferred embodiment but is not limited to same. Thus, whereas changesin capacitive reactance are compensated for by a capacitance devicelocated in the same bridge arm as the detector plate, it will beappreciated that the compensation may be obtained with a capacitivereactance located in another bridge arm. Other modifications may be madewithout departing from the scope and spirit of the present invention asdefined by the appended claims.

Iclaim:

1. An object detection system comprising:

detector means having a variable capacitance with respect to earthground and which capacitance changes in value in a given direction inresponse to the presence of an object;

oscillator means for providing a reference frequency voltage signal withrespect to earth ground;

a normally balanced capacitance bridge having four bridge arms, eachsaid bridge arm having a pair of terminals points respectively connectedto at least two other bridge arms, and at least three capacitance meanseach having a iirst and second terminal, said three iirst terminalsbeing respectively coupled to different ones of said terminal points,and said three second terminals being connected in common, an

input circuit means coupled to said oscillator means for receiving saidreference frequency voltage signal and an output circuit means forcarrying an output frequency voltage signal representative of a bridgeunbalanced condition;

said detetcor means being coupled to one of said terminal points of saidbridge so that said bridge output circuit carries a said outputfrequency voltage signal when said variable capacitance changes in valuein said given direction;

compensating means coupled to one arm of said bridge for, upon beingactuated, returning said bridge to a balanced condition; and

compensating control circuit means coupled to said bridge output circiutand to said compensating means for actuating said compensating means toreturn said bridge to a balanced condition in response to a said bridgeoutput frequency voltage signal.

2. An object detection system as set forth in claim 1, wherein saiddetector means includes electrically conductive means electricallyspaced from a reference level of earth ground potential by a dielectricmedium.

3. An object detector system as set forth in claim 2, wherein saidelectrically conductive means generally deiines a plane extendingsubstantially parallel with the plane of earth ground.

4. An object detector system as set forth in claim 3, wherein saidelectrically conductive means includes a wire mesh screen.

S. An object detection system as set forth in claim 1 wherein each saidbridge arm includes an electronic control means having an input circuitand an output circuit means, said input circuit of two of saidelectronic control means being connected to said oscillator means, saidinput circuit of the other two of said electronic control means beingadapted to be connected to a voltage supply source, and the outputcircuit means of each said control means being connected in series withone of said bridge arms.

6. An object detection system comprising:

detector means having a variable capacitance with respect to earthground and which capacitance changes in value in a given direction inresponse to the presence of an object;

oscillator means for providing a reference frequency voltage signal withrespect to earth ground;

a normally balanced capacitance bridge having four bridge arms, an inputcircuit coupled between earth ground and said oscillator means forreceiving said reference frequency voltage signal and an output circuitmeans for carrying an output frequency voltage signal representative ofa bridge unbalanced condition;

said detector means being coupled to one arm of said bridge so that saidbridge output circuit carries a said output frequency voltage signalwhen said variable capacitance changes in value in said given direction;

compensating means coupled to one arm of said bridge for compensatingfor a said change in value of said variable capacitance to return saidbridge to a balanced condition;

compensating control circuit means coupled to said bridge output circuitand to said compensating means for actuating said compensating means toreturn said bridge to a balanced condition in response to a said bridgeoutput frequency voltage signal; and,

phrase shifting means coupled to said oscillator means for shifting saidreference frequency voltage signal by and wherein said compensatingmeans includes an electrically controlled variable capacitance devicecoupled to one of said bridge arms.

7. An object detector system as set forth in claim 6,

wherein said electrically controlled variable capacitance device is thesame bridge arm to which said detector 1 16 means is coupled, andwherein said phase shifting means 2,929,998 3/ 1960 Diehl 330--29 shiftssaid reference frequency signal so that it is 90 2,983,852 5/ 1961 Gray328-5 X out of phase and lags that of said oscillator means, 3,044,6947/ 1962 Davidson et al.

whereby the direct current voltage signal represents only a change inthe capacitive reactance of said detector 5 JOHN S- HEYMAN, PrimaryEXamlUef means with respect to earth ground. U.S. C1. X.R.

Referen'cescited 323 75- 328 20s- 330 14- 331 6s 13s- 340 25s' UNITEDSTATES PATENTS 350:29 2,708,746 5/1955 shaw 340-258 10 2,922,880 1/1960Elam 328-5

